Method of making leadframe strip

ABSTRACT

A leadframe strip for use in making leaded integrated circuit packages includes a plurality of integrally connected leadframes that each have a die attach pad and first and second dam bars located adjacent to opposite first and second sides of the die attach pad, respectively. A plurality of continuous lead structures extend, uninterrupted by other structure, between opposing ones of the dam bars of horizontally adjacent leadframes. The plurality of integrally connected leadframes are arranged in a plurality of vertical columns, wherein die attach pads in one vertical column are vertically offset from die attach pads in adjacent vertical columns.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/680,499, filed Aug. 18, 2017, which is a division of U.S. patentapplication Ser. No. 15/004,208, filed Jan. 22, 2016 (now U.S. Pat. No.9,741,643), the entirety of which is hereby incorporated herein byreference.

BACKGROUND

At the beginning of a typical process for making integrated circuit (IC)packages, leadframes that are to be included in the packages areintegrally physically connected in a thin metal sheet known in the artas a “leadframe strip.” IC dies are mounted on the leadframes and thenwire bonded to leads of the leadframes, while the leadframes are instrip form. The leadframes of the strip and the dies mounted thereon arenext encapsulated in mold compound. Subsequently, each encapsulateddie/leadframe unit is separated (singulated) from the other units on theleadframe strip to provide individual IC packages. The singulationprocess for IC packages with exposed, outwardly extending leads (“leadedIC packages’) is different than the singulation process for IC packageswith leads that do not extend outwardly from the encapsulating moldcompound.

SUMMARY

A leadframe strip for use in making leaded integrated circuit packagesincludes a plurality of integrally connected leadframes that each have adie attach pad and first and second dam bars located adjacent toopposite first and second sides of the die attach pad, respectively. Aplurality of continuous lead structures extend, uninterrupted by otherstructure, between opposite ones of the dam bars of horizontallyadjacent leadframes. The plurality of integrally connected leadframesare arranged in a plurality of vertical columns wherein die attach padsin one vertical column are vertically offset from die attach pads inadjacent columns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a broken front view of a prior art lead frame strip.

FIG. 2 is a detail view of a portion of FIG. 1.

FIG. 3 is a detail front view of a prior art lead frame strip and moldplaten illustrating the flow of mold compound onto the leadframe strip.

FIG. 4 is a broken front view of an example lead frame strip.

FIG. 5 is a front detail view of interconnected lead frames of theexample lead frame strip of FIG. 1.

FIG. 6 is a front detail view of an example lead frame strip and moldplaten illustrating the flow of mold compound onto the leadframe stripof FIG. 1.

FIG. 7 is a front view of a portion of a molded leadframe strip beforepitch correction.

FIG. 8 is a front view of a portion of a molded leadframe strip afterpitch correction.

FIG. 9 is a front detail view of FIG. 7.

FIG. 10 is a front view of another example lead frame strip, aftermolding and lead separation and before pitch correction.

FIG. 11 is a front view of the example leadframe strip of FIG. 10 afterpitch correction.

FIG. 12 is a broken, detail front view of one lateral half of theleadframe strip of FIGS. 10 and 11.

FIG. 13 is an end view of an integrated circuit package singulated froma molded lead frame strip such as shown in FIG. 8 or FIG. 11.

FIG. 14 is a front view of the integrated circuit package of FIG. 13.

FIG. 15 is a flowchart of a method of making integrated circuitpackages.

DETAILED DESCRIPTION

FIG. 1 is a front view of a prior art lead frame strip 10 having ahorizontal axis 11 and a vertical axis 15. The frame of reference usedto describe the leadframe strip 10 corresponds to that of a sheet ofdrawing where a direction between the top and bottom of the sheet isreferred to as a “vertical” direction and the direction perpendicular tothe vertical direction defines a “horizontal” direction. A directionperpendicular to the plane of the page is referred to as the “depth” or“thickness” direction. The “front” of the leadframe strip 10 is shown inthe front view of FIG. 1 and is analogous to the front face of a sheetof paper.

The leadframe strip 10 comprises a pair of horizontal side rails 12 anda pair of vertical end rails 14. FIG. 2 is a detail view of a portion ofFIG. 1. As best shown by FIG. 2, the leadframe strip 10 comprises aplurality of integrally connected leadframes 16 that are positionedinside an outer periphery of the strip defined by the horizontal siderails 12 and vertical end rails 14. Each leadframe 16 has a die attachpad 18 and a plurality of outer leads 22 (or simply “leads 22”) andinner leads 23. The leads 22 extend outwardly from inner dam bars 24,which are positioned on opposite sides of each die attach pad 18. Anouter dam bar 26 is positioned midway between vertically adjacent leadframes 16. The leads 22 that extend outwardly from each inner dam bar24, extend to an associated outer dam bar 26. The distance between dambars 24 of adjacent leadframes 16 is thus approximately the combinedlength of two opposed leads 22. As shown by both FIGS. 1 and 2, theleadframes 16 are arranged in vertical columns 30, with the leadframes16 in adjacent vertical columns 30 positioned in horizontal alignment.Each pair of vertical leadframe columns 30 are positioned between avertical mold runner 32 on one side and an interior vertical rail 34 onthe other side. As illustrated by FIG. 1, a horizontally extending bone36 is positioned at a vertical midpoint of the strip 10 and providesstructural support thereto.

FIG. 3 is a detail front view of the prior art lead frame strip 10 and amold platen 40 illustrating the flow direction 42 of mold compound ontothe leadframe strip. The mold platen 40 in FIG. 3 is positioned beneaththe leadframe strip 10, depth-wise. The mold compound flows verticallypast the horizontally extending rail 12 and then through and verticallyalong each mold runner 32 and then horizontally as shown at 44 topartially cover aligned pairs of leadframes 16 positioned on oppositelateral sides of each mold runner 32. The dashed blocks shown in FIGS. 2and 3 illustrate the boundaries of integrated circuit (IC) packages thatwill ultimately be formed by processes described herein. Duringsubsequent processing the mold runners 32 and interior vertical railsand mold compound that covers these structures is cut away and scrapped(discarded). Thus, a significant amount of leadframe copper and moldcompound is lost in this process of making leaded IC packages.

FIG. 4 is a front view of an example lead frame strip 110 having ahorizontal axis 111 and a vertical axis 115 and an axis 117perpendicular to the other axes 111, 115, i.e., in the depth direction.The leadframe strip 110 has opposite horizontal rails 112 and oppositevertical rails 114. Two horizontal bones 116 extend between the verticalrails 114 dividing the strip 110 into three approximately equal heightportions. Depending upon the vertical dimension of a leadframe stripmore or fewer vertical bones may be used. The bones 116 providestructural support to the leadframe strip 110. The leadframe strip 110includes a plurality of integrally connected leadframes 118.

FIG. 5 is a front detail view of the example lead frame strip 110 ofFIG. 4. As best shown by FIG. 5 the integrally connected leadframes 118each comprise a die attach pad 120 and a plurality of leads 122. A firstand second inner dam bar 124 and 126 are positioned on opposite lateralsides of each die attach pad 120. The leadframes 118 are arranged in aplurality of vertical columns 131, 132, 133, etc. The first inner dambars 124 on the first side of each leadframe 118 are aligned in acolumn, e.g., column 131, which has a first vertical dam bar axis XX.The second inner dam bars 124 are vertically aligned along a secondvertical dam bar axis YY. A dashed square 136 around each leadframe dieattach pad 120 represents the shape of an IC package that willultimately be formed in association with each leadframe 118. The outerdam bar structure such as shown at 26 in FIG. 2 has been eliminated.Also, the leadframes 118 in one vertical column, e.g., 131 are staggeredvertically with respect to the leadframes 118 in an adjacent verticalcolumn, e.g., 132. This staggering of leadframes enables the leads 122of adjacent leadframes in adjacent columns to be interdigitated so thatthe leads of one leadframe extends from one of its dam bars to the dambar of the adjacent leadframe, eliminating the need for an outer dambar. This structure allows adjacent leadframe columns to be positionedmore closely than the prior art structure shown in FIGS. 1-3 and thusenables more leadframes to be provided in a comparable space.

FIG. 6 is a front detail view of an example lead frame strip 110 and amold 140, illustrating the flow of mold compound from exterior moldrunners 137, 139 onto the leadframe strip 110 shown in FIGS. 1 and 5.The mold 140 may be a transfer mold. Such molds are known in the art.Short length mold runners 141, 142, 143, 144, etc., are aligned withrespective ones of the plurality of vertical columns 131, 132, 133, 134,etc. These mold runners 141, etc., are positioned, depth-wise,immediately above or below one of the horizontal rails 112 and do notextend inwardly beyond the horizontal rail 112. Mold compound flows fromeach mold runner 141, etc. in direction 148 and covers the portion ofeach leadframe in each column that is positioned between axes A A andBB. Thus, in this configuration, the mold compound flows only in avertical direction and vertical mold runner structures, such as shown at32 in FIG. 1, and interior vertical rails such as shown at 34 in FIG. 1are eliminated from the leadframe strip 110. Elimination of the moldrunner structure and interior vertical rails makes more space availablefor leadframes in leadframe strip 110. Thus, the elimination of moldrunner structures and vertical rails from the inner portion of theleadframe 110, in combination with the space saving features describedin the immediately preceding paragraph, allows a leadframe strip 110 tobe provided that has a much denser leadframe configuration, i.e., moreleadframes per unit area than the prior art leadframe 10 of FIGS. 1-3.The meaning of “vertical,” as used herein, is not dependent upon whichside of the leadframe sheet is longer. Rather, the term “vertical,” asused herein, refers to the direction in which succeeding leadframes,which are linearly aligned in a column, extend.

FIG. 7 is a front view of a portion of a leadframe strip 150 aftermolding, as described above, and after conventional dam bar cutting,lead cutting, and lead forming. Such dam bar cutting and lead cutting inthe fabrication of leaded IC packages is known in the art. Accordingly,those processes will not be further described. The leadframe 150 of FIG.7 comprises molded leadframe columns 151, 152, 153 and 154. Each oddcolumn 151, 153 comprises a molded portion 171 and outwardly projectingleads 173. Each even column 152, 154 comprises a molded portion 174 andoutwardly projecting leads 176. Adjacent ones of columns 151-154 havevertically offset leadframes as described above with reference to FIGS.4-6. Odd columns 151 and 153, etc., have leadframes that are laterallyaligned. Even columns 152 and 154, etc., also have leadframes that arelaterally aligned. The leads 173 of the odd columns in FIG. 7 arevertically offset from the leads 176 of the even columns. The inventorshave discovered a method by which all columns 151-154 may be positionedwith the leads 173 and 176 and the corresponding molded leadframeportions of each column arranged in lateral alignment, as will now bedescribed.

With continued reference to FIG. 7 it may be seen that the odd columns151, 153, etc. are positioned with the lower ends 162 thereof furtherfrom the bottom rail 112 and with the upper ends 164 thereof closer tothe top rail 112 than the even leadframes 152, 154, etc. Each of theleadframe columns is connected to the lower rail 112 by lower extensions166 of the dam bar columns and to the upper rail 112 by upper extensions168 of the dam bar columns.

In FIG. 8 the odd columns 151, 153 and even columns 152, 154 have beenrelatively vertically displaced such that the leadframes thereofincluding corresponding ones of leads 171 and 176 are now laterallyaligned. In one embodiment this is done by moving odd columns 151 and153 relatively downwardly. In another embodiment it is done by movingeven columns 152, 154 relatively upwardly. In another embodiment oddcolumns are moved downwardly and even columns are moved upwardly toachieve alignment. Such upward or downward movement may be achievedthrough shortening the connecting dam bar structures at one end of acolumn and lengthening it at the other end. Such shortening andlengthening of the dam bar or associated structures may be achieved byincreasing the bend therein for shortening and by straightening the bendtherein for lengthening. (The bends in the dam bar extension structures166, 168 is not shown in FIGS. 7 and 8, but are shown and described indetail with reference to FIG. 9)

FIG. 9 is a front detail view of one lateral half of a molded column,e.g. 171, of FIG. 7, showing one dam bar extension structure 168, havinga down set portion 169, connected to one of the horizontal rails 112.The other lateral half of the molded column, e.g., 151, shown in FIG. 8,is a mirror image of the lateral half shown. By straightening the downset portion 169, the dam bar extension structure 168 may be lengthened.By further bending the down set portion 169 the structure 168 may beshortened. Thus, the associated molded column, e.g. column 151, may beselectively shifted upwardly or downwardly by increasing the bend in thedown set portion 169 at one end thereof while straightening the down setportion 169 at the other end. The dam bar or other connecting structureat each end of the column, e.g., 151, must initially be formed with somesort of bent portion in order to accomplish this upward or downwarddisplacement of the corresponding column, e.g. 151.

FIG. 10 is a front view of another example lead frame strip 210, aftermolding and lead separation and before pitch correction. FIG. 11 is afront view of the example leadframe strip of FIG. 10 after pitchcorrection. FIG. 12 is a broken, detail front view of one lateral halfof the leadframe strip of FIGS. 10 and 11. As shown by FIGS. 10 and 12,strip 210 has a first molded leadframe column 251 connected to a lowerrail 212 by mirror image dam bar extensions 266 at one end and similaror identical dam bar extensions 268 (not visible in FIGS. 10 and 11) atthe other end. FIG. 12 shows details of a structure used for increasingthe bend in dam bar 268. As shown by FIG. 12, the dam bar structure 268has a straight portion 282 adjacent the molded portion 271, which isconnected to a generally U-shaped portion 284, which is connected to arelatively straight portion 286. The relatively straight portion 286 isintegrally connected to the horizontal rail 212. A holding assemblyincludes a block member 288 and a clamping member 292 that firmly holdsstraight portion 268 in stationary relationship therewith. Adisplaceable punch member 293 may be urged against the U-shaped portion284, increasing the depth thereof and pulling the molded column 251 indirection 291 across an insert block 295. The movement of the moldedcolumn 251 in direction 291 causes a corresponding straightening of aU-shaped portion 294 of the dam bar structure 266 at the opposite end ofthe molded column 251.

As shown by FIG. 11, as a result of the displacement of column 251, thetwo columns 251 and 252 are now pitch corrected, i.e., havecorresponding leads 271 and 272 thereof in lateral alignment. Thecolumns 251 and 252 may now be singulated by one or more saw cuts (notshown in FIGS. 10 and 11) to produce singulated integrated circuitpackages 300 having molded portions 302 and lead portions 304, such asshown in FIGS. 13 and 14, which are end and rear views thereof,respectively.

FIG. 15 is a flow chart of a method of making integrated circuitpackages. The method includes, as shown at block 311, making a leadframestrip having opposite horizontally extending rails and a plurality ofintegrally connected leadframes positioned in vertical columns connectedto the horizontal rails. The method further includes, as shown at block312, placing the leadframe strip in a mold. The method also includes, asshown at block 313, injecting mold compound onto the strip from runnersending vertically outwardly of each vertical leadframe column.

Embodiments of a novel leadframe strip and methods of using such a stripto make integrated circuit packages have been described in detailherein. Alternate embodiments of such a leadframe strip and associatedmethods of making integrated circuit packages will occur to thoseskilled in the art after reading this disclosure. It is intended thatthe appended claims be construed to cover all such alternativeembodiments, except as limited by the prior art.

What is claimed is:
 1. A method of making leaded integrated circuitpackages comprising: placing a leadframe strip in a mold, the leadframestrip including: a first leadframe column and a second leadframe column,each of the first leadframe column and the second leadframe columnincluding a plurality of leadframes, each of the plurality of leadframesincluding a die attach pad of a plurality of die attach pads, theplurality of die attach pads including a first die attach pad of thefirst leadframe column and a second die attach pad of the secondleadframe column; and a plurality of leads associated with each of theplurality of die attach pads, the plurality of leads associated with thefirst die attach pad and the second die attach pad connected to a firstdam bar associated with the first die attach pad and a second dam barassociated with the second die attach pad, the first and second dam barsconnected together using a portion of the leadframe strip, the portionaligned in a first direction; wherein the first die attach pad and thesecond die attach pad are offset from each other in a second direction,the second direction being at an angle with respect to the firstdirection; and injecting mold compound onto the leadframe strip.
 2. Themethod of claim 1, wherein said injecting comprises injecting the moldcompound in the second direction.
 3. The method of claim 1 wherein saidinjecting comprises covering at least portions of plurality of dieattach pads, and the plurality of leads, with mold compound.
 4. Themethod of claim 1 further comprising laterally separating the firstleadframe column from the second leadframe column.
 5. The method ofclaim 1 further comprising performing pitch correction by moving thefirst leadframe column or the second leadframe column in the seconddirection.
 6. The method of claim 1 further comprising performing pitchcorrection by moving the first leadframe column or the second leadframecolumn upwardly in the second direction.
 7. The method of claim 1further comprising performing pitch correction by moving the firstleadframe column or the second leadframe column downwardly in the seconddirection.
 8. The method of claim 5 wherein said performing pitchcorrection comprises bending a structure associated with the first dambar or the second dam bar.
 9. The method of claim 8 wherein saidperforming pitch correction comprises bending the structure to move thefirst leadframe column or the second leadframe column in the seconddirection.
 10. The method of claim 8 wherein said bending comprisespunch bending the structure.
 11. The method of claim 1 furthercomprising singulating the leadframe strip to separate the firstleadframe column from the second leadframe column.
 12. A method ofmaking leaded integrated circuit packages comprising: placing aleadframe strip in a mold, the leadframe strip including: a plurality ofconnected leadframes arranged in a plurality of leadframe columns; and aplurality of die attach pads, each of the plurality of connectedleadframes associated with one of the plurality of die attached pads,wherein the plurality of die attach pads in two adjacent columns areoffset from each other; and injecting mold compound onto the leadframestrip through the at least one mold runner.
 13. The method of claim 12,wherein injecting mold compound comprises injecting the mold compound tothe mold runner that is aligned with a die attach pad at a bottom ofeach of the plurality of leadframe columns.
 14. The method of claim 12,wherein injecting mold compound comprises injecting the mold compound ina first direction.
 15. The method of claim 12 further comprisinglaterally separating the columns by cutting the set of leads connectingthe two adjacent columns.
 16. The method of claim 12 further comprisingsingulating the columns by performing a plurality of cuts extendingacross the columns in the first direction.